In Search of Subsurface Oceans Within the Uranian Moons

Abstract

The Galileo mission to Jupiter discovered magnetic signatures associated with hidden subsurface oceans at the moons Europa and Callisto using the phenomenon of magnetic induction. These induced magnetic fields originate from electrically conductive layers within the moons and are driven by Jupiter's strong time‐varying magnetic field. The ice giants and their moons are also ideal laboratories for magnetic induction studies. Both Uranus and Neptune have a strongly tilted magnetic axis with respect to their spin axis, creating a dynamic and strongly variable magnetic field environment at the orbits of their major moons. Although Voyager 2 visited the ice giants in the 1980s, it did not pass close enough to any of the moons to detect magnetic induction signatures. However, Voyager 2 revealed that some of these moons exhibit surface features that hint at recent geologically activity, possibly associated with subsurface oceans. Future missions to the ice giants may therefore be capable of discovering subsurface oceans, thereby adding to the family of known “ocean worlds” in our Solar System. Here, we assess magnetic induction as a technique for investigating subsurface oceans within the major moons of Uranus. Furthermore, we establish the ability to distinguish induction responses created by different interior characteristics that tie into the induction response: ocean thickness, conductivity and depth, and ionospheric conductance. The results reported here demonstrate the possibility of single‐pass ocean detection and constrained characterization within the moons of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection and trajectory design for future missions to Uranus.

A perturbation method for evaluating the magnetic field induced from an arbitrary, asymmetric ocean world analytically

“… Khurana et al, 2002 ; Hand and Chyba, 2007 ) have focused on the information contained only in the amplitude while treating the ocean as a single, uniform conducting layer. As elaborated by Vance et al (2021) and Cochrane et al (2021) , the amplitude and phase information contained in cause this quantity to occupy specific regions of the complex plane for different interior structure models. This structure-dependent behavior restricts degeneracies between models to single points, in contrast to the lines traced by degenerate response amplitudes A ( e.g.…”

“…For Europa, the maximum ratio is even smaller, at about 30 km/1530 km ≈ 0.02 ( Billings and Kattenhorn, 2005 ). The excitation moments experienced by icy moons in the solar system range from 7 nT for Triton up to about 300 nT for Miranda ( Cochrane et al, 2021 ). The largest differences in magnetic field at the surface of these bodies from the first-order term can be about 10% for highly conducting, maximally asymmetric oceans.…”

“…This demonstrated methodology will be deployed by the planned Europa Clipper and JUICE missions to characterize the oceans of these bodies. The same technique also shows promise for the investigation of the large moons of Uranus and Neptune ( Khurana, 2019 ; Weiss et al, 2021 ; Cochrane et al, 2021 ; Arridge and Eggington, 2021 ); magnetometer investigations are likely to play a central role in future missions to these moons, too.…”

The science case for spacecraft exploration of the Uranian satellites: Candidate ocean worlds in an ice giant system

“…Relatively less is known about the interiors of the classical moons, and measuring magnetic induction with a magnetometer is critical for determining whether they possess subsurface oceans (Cochrane et al 2021;Weiss et al 2021). Other measurements are needed to investigate the internal structures and bulk compositions of these moons, which is critical for improving our understanding of their formation and evolution.…”

Single‐ and Multi‐Pass Magnetometric Subsurface Ocean Detection and Characterization in Icy Worlds Using Principal Component Analysis (PCA): Application to Triton

“…Although limited data exists, many moons of the gas giants are thought to possibly contain hidden oceans. The five large Uranian moons (Miranda, Ariel, Umbriel, Titania, and Oberon) all in some form exhibit surface features consistent with a subsurface liquid ocean (e.g., Croft & Soderblom, 1991; Schenk et al., 2020), and also reside in a time‐varying magnetic environment which enables magnetic induction detection (Arridge & Eggington, 2021; Cochrane et al., 2021; Weiss et al., 2021). Neptune's moon Triton, the focus of this article, is a particularly compelling mission target with numerous characteristics consistent with the presence of a subsurface ocean.…”